Process of purifying and impregnating cellulosic spacers for electrical condensers



Oct. 19, 1954 BURNHAM 2,692,210

PROCESS OF PURIFYING AND IMPREGNATING CELLULOSIC SPACERS FOR ELECTRICALCONDENSERS Filed Dec. 10. 1949 JOHN BURNHAM,

INVENTOR.

BY awh- I gifi Patented Oct. 19, 1954 PROCESS OF PURIFYING ANDIMPREGNAT- ING CELLULOSIC SPACERS FOR ELECTRI- CAL CONDENSERS JohnBurnham, Williamstown, Mass., assignor to Sprague Electric Company,North Adams,

Mesa, a corporation of Massachusetts Application December 10, 1949,Serial No. 132,345

(Gl. 1l7- l8) 6 Claims. 1

My present invention relates to the purification of cellulosic spacermaterials. More particularly, it concerns a method of purifyingregenerated cellulose films to make them specially adapted for use inthe fabrication of electrolytic capacitors.

The use of a regenerated cellulose film as a spacer material forelectrolytic condensers has been repeatedly suggested in the patentliterabelow 25 parts per million by Weight, forcing an electrolyticcondenser electrolyte through said roll in the same axial direction, andunwinding said regenerated cellulose film from said roll. According toanother limited embodiment of the invention there is employed a processwhich comprises convolutely winding at least two regenerated cellulosefilms with at least two aluminum electrodes and at least two porouspaper ture. Unfortunately, however, successful appli- 1O spacers into acylindrical roll, whereby each re cation of the regenerated cellulosefilm to comgenerated cellulose film is separated from the mercialmanufacturing operations, particularly other said films by a porouspaper spacer and in the production of high capacity electrolytic analuminum electrode, forcing boiling water condensers, has not beenpossible. This situathrough said roll in an axial direction until thetion results from the contamination of the regeneffiuent liquid has achloride ion content suberated cellulose during its manufacture withstantially less than 25 parts per million and forcchloride ions,sulphate ions and other ionic irning an electrolytic solution in thesame axial dipurities. Such impurities in the film may lead rectionthrough said roll until said roll is satuto corrosion and failure ofelectrolytic capacitor rated with said electrolyte. structures in whichthe films are incorporated. According to my invention, I have devised aIn the wet state following extrusion, regenerated novel and simpleprocess for removing chloride, cellulose is extremely fragile and may besubsulphate and related ionic impurities from regenjected only withgreat difiiculty to thorough erated cellulose films without encounteringthe washing, in order to remove such ionic impurities. cost andequipment difficulties heretofore met in It has been suggested thatdialysis may be emthe purification process. According to one employed toremove impurities of various kinds. bodiment of the invention boilingwater or some While this process is to some extent successful, othersolvent in which the ionic impurities are it requires extremelyexpensive and massive procsoluble is forced axially through a convoluteroll essing equipment for efficiency in operation. consisting of theregenerated cellulose film and of It is an object of the presentinvention to a contacting porous spacing material, such as overcome theforegoing and related disadvantages 50 paper, cloth, glass, mat, etc.The axial washing of the regenerated cellulose films. A further obprocess is conducted only for the length of time lect is to produceimproved cellulosic spacer manecessary to reduce the chloride content ofthe terials for electrical devices. A still further obeffluent solventto the maximum desired, e. g. ject is to provide a simple andinexpensive procqr about 25 parts per million in the case of regenessfor producing regenerated cellulose film suberated cellulose films foruse in electrolytic constantially free from ionic impurities. Additionaldensers as spacer materials. objects will become apparent from thefollowing According to one of the limited embodiments description andclaims. of the invention the washing process may he fol- These objectsare attained in accordance with lowed by a treatment with a liquid whichwill be th present invention by employing a pur fication retained and/orabsorbed in the regenerated cel- Process that comprises convolutelyWinding a lulose film to act as a plasticizer and/or e1ectrosheet ofregenerated cellulose spacing material 1m F l a l ti of ammgnium th aContacting sheet of porou Spacing Illaieborate in glycol may be passedthrough the F m a f51indrica1r11 frcing a Solvent washed roll ofregenerated cellulose film and ionic impurities through said roll in anaxial di porous paper. Thereafter the T011 may be um rection, unwindingsaid You and drying the wound and optionally the regenerated celluloseSulting p regenerated Cellulose fi In film may be dried prior toincorporation in caaccordance with one of the limited embodiments acit rassemblies, of t invention there is p y a p es According to one of thepreferred embodiments which comprises convolutely winding a thin, reofmy invention the wash roll consists of an generated cellulose film witha contacting porous electrolytic capacitor assembly in which the paperinto a cylindrical roll, forcing boiling water spacing materialseparating the two aluminum through said roll in an axial directionuntil the electrodes comprises at least one layer of regenefiluent waterpossesses a chloride concentration erated cellulose and at least onecontacting layer of a porous paper material, such as linen paper orkraft capacitor paper. If porous aluminum electrodes, such as sprayedaluminum on gauze electrodes, are used, it is sometimes possible toeliminate the porous paper spacer. By this process it is possible towash the cellulose spacer to a safe chloride content and subsequently toimpregnate the wound capacitor with the operating electrolyte. Normallythe aluminum electrodes are formed with an oxide film prior toincorporation in the wash roll.

The regenerated cellulose film obtained in accordance with my inventionis useful not only as a spacing material for electrolytic capacitors,but also as a dielectric spacer for electrostatic (non-electrolytic)condensers. In such instances, the regenerated cellulose has a muchhigher insulation resistance and results in capacitors with lowerleakage currents than corresponding capacitors made with conventional,electrical grade, regenerated cellulose films.

The regenerated cellulose employed in accordance with the invention maybe made in the usual manner without special processing equipment orcontrol. Other polar resinous films are contemplated for use inaccordance with the present teachings, provided that appropriate washsolvents, wash temperatures and rates are employed.

When the basic washing process of the invention is to be practiced, itis generally desirable to wind up the contacting sheet of porous spacingmaterial as part of the finishing operation in the manufacture of theregenerated cellulose film. The wide rolls e. g. using 56" paper sheetsmay be subjected to the treating process without further handling. If acloth or fabric spacer is used, it may, of course, be reused untilfailure occurs, for example, tearing, fraying, etc.

In addition to water, mixtures of water and polyhydroxy compounds, suchas glycol and glycerine, may be employed as solvents. It is apparentthat numerous other solvents may be employed dependent upon the ionicimpurities to be removed, type of film to be processed, etc. The rate ofsolvent flow across the regenerated cellulose surface, the diffusionrate of ions from from the inner parts of the film, the temperature ofthe solvent and film are determining factors in the processing control.While I am not fully aware of the exact phenomenon involved, I believethat the primary action is a diffusion controlled process and,therefore, depends upon the diffusion rate for chloride or other ionswhich may be present within the regenerated cellulose film.

Reference may be made to the appended drawing which illustrates asimplified cross section of a typical washing apparatus suitable forcarrying out the process of my invention. Convolutely wound roll 9consists of a porous spacer sheet IE) and a regenerated cellulose filmll. This roll is fitted into the container !2. Cap i3 is attached to awash water supply. The wash water [4 flows axially through the section 9in the direction of the arrows, forming an efliuent i5 which isexhausted. A conductivity cell 16 is connected to bridge 11, as aprocess control for the amount of ionic impurities in the emuent i5. Itis generallypreferable'to force wash water [4 through section 9 by meansof simple hydrostatic pressure. However, it is also possible to pull thewash water through the section by means of a vacuum pump.

In most instances involving-large rolls of an- 4 preciable width it isdesirable periodically to reverse the axial direction of the washsolution. In this way the diffusion is equalized at each end of thesection.

As a specific example of the practice of my invention, 0.0005"cellophane of 12 width was convolutely wound with 0.003" porous linenpaper on a 1" core to a total diameter of 5". The core was stoppered andthe wound roll placed in a column. Boiling tap water was passed throughthe wound roll in an axial direction at a rate of 2 liters per minutefor 30 minutes. Following this, a liter of boiling distilled water waspassed through the section in the same axial direction. Following thedistilled water rinse, 500 cc. of a solution of 25% ethylene glycol inwater was passed through the roll. The regenerated cellulose film wasunwound and dried at 70 C. Prior to the washing operation theregenerated cellulose had a chloride ion concentration of 250 parts permillion. After the washing process the chloride ion content was only 15parts per million. The glycol was added as a plasticizer and electrolytesolvent for the film.

It should be noted that the final treatment with an aqueous glycolsolution described in the foregoing specific example is ordinarilyomitted, when electrostatic spacing material is being produced.

For some purposes the contacting sheet of porous spacing material may becomposed of metallic cloth or screen. The material employed for thispurpose is not critical, provided that it is porous in nature andchemically inert, i. e., does not impart ionic impurities to theregenerated cellulose film.

My new purification process requires a minimum quantity of solvent suchas distilled water. It provides the user of the highly purified,regenerated cellulose film with a simple and inexpensive means forproducing such film from the commercially available regeneratedcellulose films containing an excessive amount of ionic impurities. Ifthe manufacturer of the regenerated cellulose film should try to removesuch ionic impurities, this would require several additional washings, amuch greater amount of floor space and a great quantity of distilledwater. This would greatly increase the cost of manufacture, and therelatively small demand for the highly purified, regenerated cellulosefilm would hardly justify the heavy investment for the additionalequipment that would be required to include such purification in theregular production of the film.

As many apparently widely diiferent embodiments of this invention may bemade without departing from the spirit and scope hereof, it is to beunderstood that the invention is not limited to the specific embodimentshereof except as defined in the appended claims.

I claim:

1. A purification process which comprises convolutely winding a sheet ofregenerated cellulose having soluble impurities with a contacting sheetof porous spacing material into a cylindrical roll, and forcing asolvent for the impurities through said roll in an axial direction tocause the impurities to diffuse out of the regenerated cellulose sheetand into the solvent.

2. A purification process which comprises convolutely winding a thin,incompletely washed" regenerated cellulose filrn with a contactingporous paper into-a'- cylindrical roll, forcing boil- 5 ing waterthrough said roll in an axial direction until the effluent waterpossesses a chloride concentration below 25 parts per million by weight,forcing an electrolytic condenser elec trolyte through said roll in thesame axial direction to saturate the resulting film with saidelectrolyte, and unwinding said regenerated cellulose film from saidroll.

3. A purification process which comprises convolutely winding at leasttwo incompletely washed regenerated cellulose films with at least twoelectrode foils and at least two porous paper spacers into a cylindricalroll having each regenerated cellulose film separated from the other ofsaid films by a porous paper spacer and an electrode foil, forcingboiling water through through said roll in an axial direction until theefliuent liquid has a chloride ion content substantially less than 25parts per million and forcing an electrolytic capacitor electrolyte inthe same axial direction through said roll until said roll is saturatedwith said electrolyte.

4. A process which comprises convolutely winding a sheet of regeneratedcellulose having soluble impurities with a contacting sheet of porousspacing material into a cylindrical roll and forcing an aqueousaliphatic polyhydroxy solvent for the impurities through said roll in anaxial direction to cause the impurities to diffuse out or" theregenerated cellulose sheet and into the solvent.

5. The process of claim 4 wherein the polyhydroxy compound is glycol.

6. The process of claim 4 wherein the polyhydroxy compound is glycerine.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 643,234 Post Feb. 13, 1900 719,973 Wotton Feb. 3, 19031,315,763 Dickey Sept. 9, 1919 1,807,375 Boyer May 26, 1931 1,932,789Kline Oct. 31, 1933 1,981,352 Fruth Nov. 20, 1934 2,095,611 McNutt Oct.12, 1937 2,158,931 Collins May 16, 1939 2,199,447 Ruben May 7, 19402,253,507 Clark Aug. 26, 1941 2,290,163 Brennan July 21, 1942 2,460,282Gardner Feb. 1, 1949 FOREIGN PATENTS Number Country Date 670,853 FranceDec. 5, 1929

3. A PURIFICATION PROCESS WHICH COMPRISES CONVOLUTELY WINDING AT LEAST TWO INCOMPLETELY WASHED REGENERATED CELLULOSE FILMS WITH AT LEAST TWO ELECTRODE FOILS AND AT LEAST TWO POROUS PAPER SPACER INTO A CYLINDRICAL ROLL HAVING EACH REGENERATED CELLULOSE FILM SEPARATED FROM THE OTHER OF SAID FILMS BY A POROUS PAPER SPACER AND AN ELECTRODE FOIL, FORCING BOILING WATER THROUGH THROUGH SAID ROLL IN AN AXIAL DIRECTION UNTIL THE EFFLUENT LIQUID HAS A CHLORIDE ION CONTENT SUBSTANTIALLY LESS THAN 25 PARTS PER MILLION AND FORCING AN ELECTROLYTIC CAPACITOR ELECTROLYTE IN THE SAME AXIAL DIRECTION THROUGH SAID ROLL UNTIL SAID ROLL IS SATURATED WITH SAID ELECTROLYTE. 